In recent kernels an option called arptables popped up. Like iptables and ipv6tables it does pretty much what the name suggests. However, I can't find any userland tools for it and this message suggests there aren't any.

After reading the code it seems resonably easy to do. Unless someone beats me to it, I might give it a shot

DTCP

... but before I do, I'm going to give libdtcp a crack. DTCP is the protocol used in Coderman's Alpine and is designed for double NATed hosts with loose UDP-NAT rules. Watch this space.

Java-SSH

Since Mozilla is still being clipboard brain-dead I'm typing URLs into vim by hand. This means that I mess up some of them (since I'm too lazy to check) and, sure enough, I messed up the link to JSCH. Atsuhiko Yamanaka was kind enough to mail me and point it out. (now fixed)